Generator dispatching or load shedding control method and system for microgrid applications

ABSTRACT

A microgrid power generation system includes a plurality of generators having a plurality of different rated capacities and a plurality of distribution nodes, at least some of the distribution nodes being powered by the generators. A grid is formed by the distribution nodes, the grid includes a system frequency. A plurality of loads are powered by the grid through the distribution nodes, the loads have a power demand. A processor includes a plurality of efficiency bands, each of the efficiency bands being for a corresponding one of the generators and including a plurality of generator switching points based upon droop of the system frequency and the power demand of the loads. The processor is structured to operate the generators and the loads under transient conditions based upon the efficiency bands.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from and claims the benefit of U.S.Provisional Patent Application Ser. No. 61/710,905, filed Oct. 8, 2012,which is incorporated by reference herein.

BACKGROUND

1. Field

The disclosed concept pertains generally to power generation systemsand, more particularly, to microgrids, such as, for example, distributedgeneration power systems. The disclosed concept further pertains togenerator dispatching control methods for microgrids. The disclosedconcept also pertains to load shedding control methods and systems.

2. Background Information

Fuel consumption and power reliability are two major concerns for powergeneration applications. In order to have a robust solution, the trendin power generation systems is developing toward distributed generation(DG) which includes conventional grid connect, conventional fossil fuelgeneration and renewable energy resources.

A typical approach for generator dispatching is to add or turn off agenerator based on frequency droop characteristics of plural generatorsas a function of the grid load level in order to maintain system powerreliability. As shown in FIG. 1, generator dispatching frequencythresholds are typically set within a normal load range. However, thisapproach does not always guarantee maximum efficiency and is notscalable for relatively larger intelligent distributed power generationsystems. Moreover, if renewable energy resources are used, then thecontrol strategy needs to be robust in order to provide desiredflexibility.

In the generator dispatching control strategy of FIG. 1, which does notuse an energy storage system, plot 2 is for both 30 kW and 60 kWgenerators (not shown) being on, while plot 4 is for only the 60 kWgenerator (not shown) being on. The plots 2,4 show per unit (pu)frequency (f) on the vertical axis and power output (kW and per unitpower) on the horizontal axis. In the plot 2, when the power outputdecreases to 27 kW, or the frequency increases to 0.994 pu, the 30 kWgenerator is turned off at 6. Later, when the power output increases to48 kW in plot 4, or the frequency decreases to 0.984 pu, the 30 kWgenerator is turned on at 8. The 60 kW generator keeps running as amaster generator, turns on the 30 kW generator when the load exceeds asuitable threshold (e.g., without limitation, 80% of 60 kW), and turnsoff the 30 kW generator when load is below a suitable threshold (e.g.,without limitation, 30% of 90 kW).

FIG. 2 illustrates demand dispatching through a simulation. The plottedsignals represent the system frequency that is drooping based on asimulated load and based on the capacity slope of the operatinggenerator. The dotted line is an indication of the operating state of asupporting generator. When the dotted line is “high” on the plot, asecond supporting generator is online. When the dotted line is “low”,the second supporting generator is offline. At about time 3.0 asignificant load step occurs. This load step draws greater power thanthe prescribed limit for the single operating generator. Additionalgeneration is then applied causing the frequency to recover. At abouttime 7.0, the excess load is removed and the frequency begins to recoverforcing the additional generation to turn off. As the transient from theload step continues, the frequency droops at 10 below the transitionthreshold and the additional generation is reapplied. After thefrequency settles, following the load step transient, the additionalgeneration is again commanded offline.

Hence, there is a need to prevent this generator cycling issue.

There is room for improvement in microgrids, such as, for example,distributed generation power systems.

There is also room for improvement in generator dispatching controlmethods for microgrids.

There is further room for improvement in load shedding control methodsfor microgrids.

SUMMARY

These needs and others are met by aspects of the disclosed concept.

As one aspect of the disclosed concept, a generator dispatching controlmethod for a microgrid comprises: employing a plurality of generatorshaving a plurality of different rated capacities; employing a pluralityof distribution nodes; powering at least some of the distribution nodesby the generators; forming a grid by the distribution nodes, the gridincluding a system frequency; powering a plurality of loads by the gridthrough the distribution nodes, the loads having a power demand;operating by a processor the generators and the loads under transientconditions based upon a plurality of efficiency bands; and employingeach of the efficiency bands for a corresponding one of the generators,each of the efficiency bands including a plurality of generatorswitching points based upon droop of the system frequency and the powerdemand of the loads.

As another aspect of the disclosed concept, a microgrid power generationsystem comprises: a plurality of generators having a plurality ofdifferent rated capacities; a plurality of distribution nodes, at leastsome of the distribution nodes being powered by the generators; a gridformed by the distribution nodes, the grid including a system frequency;a plurality of loads powered by the grid through the distribution nodes,the loads having a power demand; and a processor including a pluralityof efficiency bands, each of the efficiency bands being for acorresponding one of the generators and including a plurality ofgenerator switching points based upon droop of the system frequency andthe power demand of the loads, the processor being structured to operatethe generators and the loads under transient conditions based upon theefficiency bands.

As another aspect of the disclosed concept, a load shedding controlmethod for a microgrid comprises: employing a plurality of generators;employing a plurality of distribution nodes; powering at least some ofthe distribution nodes by the generators; forming a grid by thedistribution nodes, the grid including a system frequency; powering aplurality of loads by the grid through the distribution nodes; andmeasuring the system frequency of the grid, comparing the measuredsystem frequency to a frequency threshold, and responsively shedding bya processor a number of the loads from the grid or prohibitingadditional load to the grid.

As another aspect of the disclosed concept, a load shedding systemcomprises: a plurality of generators; a plurality of distribution nodes,at least some of the distribution nodes being powered by the generators;a grid formed by the distribution nodes, the grid including a systemfrequency; a plurality of loads powered by the grid through thedistribution nodes; and a processor including a frequency-based loadshedding routine structured to measure the system frequency of the grid,compare the measured system frequency to a frequency threshold, andresponsively shed a number of the loads from the grid or prohibitadditional load to the grid.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the disclosed concept can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a plot of generator per unit frequency versus power for twoconfigurations of 30 kW and 60 kW generators, and a single 60 kWgenerator including transitions between the two configurations.

FIG. 2 is a plot of simulated system per unit frequency versus time forvarious loads and transitions between two generator operating states.

FIG. 3 is a plot of simulated system per unit frequency versus time forvarious loads and transitions between two generator operating states inaccordance with embodiments of the disclosed concept.

FIG. 4 is a plot of generator per unit frequency versus power for 30 kWand 60 kW generators for which control logic with multiple dispatchingpoints, but without an energy store, is employed in accordance withanother embodiment of the disclosed concept.

FIG. 5 is a flow chart of time-based control logic corresponding to theplot of FIG. 4.

FIG. 6 is a plot of generator per unit frequency versus power for 30 kWand 60 kW generators for which control logic with multiple dispatchingpoints and an energy store is employed in accordance with anotherembodiment of the disclosed concept.

FIG. 7 is a flow chart of time-based control logic corresponding to theplot of FIG. 6.

FIG. 8 is a plot of generator per unit frequency versus power for anumber of 30 kW generators and zero or one 60 kW generator for multipledispatching points, but without an energy store, in accordance withanother embodiment of the disclosed concept.

FIG. 9 is a plot of system efficiency versus power for a number of 30 kWgenerators and zero or one 60 kW generator for multiple dispatchingpoints, but without energy store, in accordance with another embodimentof the disclosed concept.

FIG. 10 is a simplified block diagram of a system including twogenerators, an energy storage system and a DC/AC inverter, a grid formedby plural distribution nodes, and plural loads in accordance withanother embodiment of the disclosed concept.

FIG. 11 is a control block diagram of the system of FIG. 10 including adistribution manager for each of the distribution nodes.

FIG. 12 is a plot of per unit frequency versus per unit power for a 2%droop.

FIG. 13 is a block diagram of a load shedding algorithm including anunder-frequency load shedding relay in accordance with anotherembodiment of the disclosed concept.

FIG. 14 is a plot of generator per unit power versus time for a systemincluding 30 kW and 60 kW generators in which the 60 kW generator tripsoffline in accordance with another embodiment of the disclosed concept.

FIG. 15A is a plot of generator per unit frequency and filtered andsampled frequency versus time for the load shedding algorithm of FIG.13.

FIG. 15B is a plot of a normalized accumulator and a (F−F_(THRESH))²function versus time for the load shedding algorithm of FIG. 13.

FIG. 15C is a plot of digital signals used to shed and restore loadsversus time for the load shedding algorithm of FIG. 13.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the term “number” shall mean one or an integergreater than one (i.e., a plurality).

As employed herein, the term “processor” shall mean a programmableanalog and/or digital device that can store, retrieve, and process data;a control; a controller; an intelligent distribution manager; acomputer; a workstation; a personal computer; a microprocessor; amicrocontroller; a microcomputer; a central processing unit; a mainframecomputer; a mini-computer; a server; a networked processor; or anysuitable processing device or apparatus.

As employed herein, the term “microgrid” shall mean a MicroGrid, aSmartGrid, distributed generation (DG), on-site generation, dispersedgeneration, embedded generation, decentralized generation, decentralizedenergy or distributed energy, which generates electricity from aplurality of the same or different energy sources. Non-limiting examplesof such energy sources include diesel generation, wind energy, solarenergy, and energy storage systems, such as a number of batteries thatare electrically connected to other energy sources by a bi-directionalDC/AC inverter. Although not required, a microgrid can allow collectionof energy from relatively many energy sources and may give lowerenvironmental impacts and improved security of power supply. Typically,although not required, a microgrid is islanded or electricallydisconnected from a utility grid.

As employed herein, the statement that two or more parts are “connected”or “coupled” together shall mean that the parts are joined togethereither directly or joined through one or more intermediate parts.Further, as employed herein, the statement that two or more parts are“attached” shall mean that the parts are joined together directly.

The disclosed concept is described in association with dieselgeneration, although the disclosed concept is applicable to a wide rangeof energy sources for a microgrid.

In FIG. 2, the unstable response to the load transient can be mitigatedwith an appropriate delay. FIG. 3 shows a frequency response at 12 to aload transient, but with a suitable delay to prevent generator cycling.Like FIG. 2, the same load transient occurs at about time 3.0, as shownby the dotted line at 14, and the additional generation is broughtonline. Later, at about time 7.0, the load transitions offline, but thetransient doesn't cause the additional generation to go offline untilthe transient is over and the frequency has stabilized. This eliminatesthe undesired generator cycling that was discussed above in connectionwith FIG. 2. As the number of sources and loads are increased, asuitable delay time is needed to prevent undesired cycling.

FIG. 2 is a simple simulation that illustrates frequency response to arelatively large load transient. Any substantial load may have theeffect of drooping the frequency to a level that causes additionalgeneration to be brought online. The plot indicates the potential forload transients to cause generators to start and stop undesirably. Toprevent the errant starting of generators in response to loadtransients, a suitable response delay is applied to the source and loadmanagement. This response provides time for the frequency to “settle” ata new operating point before additional generation is brought on line.For contingencies where the frequency droops at a relatively high ratefrom an overload or another power system fault, the disclosed conceptprovides load management that sheds loads in order to prevent systemcollapse. Reconnection of loads is performed following frequencyrecovery in a corresponding suitable manner to ensure the system canprovide service without instability.

The disclosed concept provides a control strategy for distributedgenerators in order to provide maximum fuel efficiency while maintainingpower reliability of an energy storage system. The disclosed systemsemploy demand-based dispatching and can be extended to cover all loadinglevels for maximum energy efficiency. An increase in reliability orpower quality can be obtained by operating plural power sources andloads autonomously in transient conditions, namely, when the system isdisturbed in the microgrid.

A frequency-based load shedding algorithm for a microgrid operating inan islanded mode is also disclosed. This employs frequency droop forreal power sharing. This algorithm implements emergency (e.g., a numberof loads are shed immediately in order to prevent a number of generatorsfrom stalling), as well as non-emergency, load shedding, and addressesdifferences in frequency-based load shedding for a utility applicationversus a microgrid application.

Example 1

Diesel generation, for example, runs at maximum fuel efficiency whenoperated at rated power. Hence, in order to maximize overall system fuelefficiency, diesel DG needs to be operated at or near its nominal loadrating as often as possible. There are two primary strategies availableto accomplish this goal from a microgrid control architecture: (1)generator dispatching; and (2) peak shaving of generators. Theefficiency bands disclosed herein are intended to provide the highestoperational efficiency for diesel DG sourcing power to the microgrid,while ensuring reserve capacity in the diesel DG or a battery interfacemodule (BIM) when connected to prevent a contingency from overloadingthe running diesel DG from an unanticipated surge in demand.

Example 2

A BIM is, for example, a three-phase, four-wire grid tie inverter ratedfor the continuous capacity of the corresponding energy storage system(e.g., without limitation, rated at 6 kWH), and a transient capacity ofthe diesel DG that it is paired with. The BIM interfaces an energystorage system with the microgrid by providing equivalent voltage,frequency and phase of the diesel DG sourced power. The BIM functions asa complementary source for peak shaving for the diesel DG by addingtransient demand power when the diesel DG is running near 100% capacity.This allows continued operation at the highest diesel DG efficiency fora relatively short time without transferring the demand to a higherrated diesel DG running at less than capacity and incurring additionallosses in efficiency. At extreme low demand, the BIM is available toassume sourcing for the entire microgrid, thereby allowing the diesel DGto cycle off. This allows for the stored energy accumulated during powercycling to be used in lieu of the diesel DG continuing to burn fuel at arelatively very low efficiency.

Example 3

Generator dispatching consists of having just the right amount of dieselDG operating on the system to ensure that the generation capacity andload demands are balanced at any given time. This strategy implies thatdiesel DG of different ratings are on stand-by to be turned on or off asneeded based on the present load demand. To have the effective controlfor stable and efficient paralleling of widely distributed diesel DG,voltage and frequency droop control is employed as modified by the useof efficiency bands as will be described.

Example 4

The second source management strategy consists of peak shaving thegenerators on the system such that the diesel DG operate at or close to100% load when active, supplying the load and charging an energy storagesystem simultaneously, and then turning off the diesel DG and supplyingthe loads from the energy storage system. Additional diesel DG may runin parallel to meet the demand beyond the capacity of the energy storagesystem. This strategy ensures that anytime a diesel DG is turned on, itis operated at or close to rated capacity. The efficiency bands for therunning diesel DG are modified dynamically based the availability of themicrogrid-tied energy storage system. Based on the efficiency bands, theenergy storage system will support the running diesel DG to delaystarting additional units as well as supporting transient peak load. Incases where the load exceeds the capacity of the running diesel DG, theenergy storage system will carry the additional load while higher dieselDG capacity is brought online.

Example 5

The control logic of the system includes a plurality of switching pointsto keep plural generators running close to the highest efficiencyoperating region (e.g., without limitation, an example 90% efficiencyband 20 is shown in FIG. 4 for both of the example 30 kW and 60 kWgenerators (not shown)). The plot 22 shows the results of control logicwith multiple dispatching points, but without an energy store.

For example, as shown at 24, the 30 kW generator is turned on at 100% ofthe rating of the 60 kW generator (i.e., 60 kW in this example). Then,the 30 kW generator is turned off, at 26, when the load is at 90% of therating of the 60 kW generator alone (i.e., 54 kW in this example). Atsomewhat less than 30 kW (e.g., 27 kW in this 90% efficiency bandexample), the 30 kW generator is turned on, at 28, and after apredetermined time (e.g., without limitation, 3 seconds), the 60 kWgenerator is turned off at 30. When the load increases to 30 kW, the 60kW generator is turned on, at 32.

Example 6

FIG. 5 shows a flow chart of time-based logic 50 for the disclosedconcept, which can be executed by a suitable processor (e.g., withoutlimitation, the control 230 of FIG. 11; a supervisory controller (notshown); an intelligent distribution manager (e.g., 222,224 of FIG. 11)).In the time-based logic 50, efficiency bands, thresholds, droop slopesand delays are preferably adjustable parameters. For example, t_init isthe initial generator start-up time, t_dwell is the generatordispatching dwell time, t_trans is the generator dispatching transitiontime, and t_inv is the generator dispatching inverter time (see FIG. 7).

The logic 50 starts at 54. During initialization, at 56, the 60 kWgenerator is turned on, and the 30 kW generator is turned off. A timer(not shown) is initialized to zero and is started. Then, after asuitable predetermined time, t_init (e.g., without limitation, 5seconds; any suitable time), execution resumes at 58, which is the“normal” load state (e.g., 30 kW to 60 kW in this example) with only the60 kW generator turned on. Here, the timer is initialized to zero and isrestarted. Then, after a suitable predetermined time, t_dwell (e.g.,without limitation, 3 seconds; any suitable time), execution resumes at60.

At 60, if the frequency (freq)<0.980, then a “high” load state (e.g., 60kW to 90 kW) is entered at 62 with both 30 kW and 60 kW generatorsturned on. Here, the timer is initialized to zero and is restarted.Then, after a suitable predetermined time, t_dwell (e.g., withoutlimitation, 3 seconds; any suitable time), execution resumes at 64. At64, if the frequency (freq)>0.988, then the “normal” load state isreentered at 58 with only the 60 kW generator turned on and the 30 kWgenerator turned off. Otherwise, the test at 64 is repeated.

On the other hand, if the test at 60 is false, then at 66, if thefrequency (freq)>0.991, then a transition state or “normal to light”state is entered at 68 with both of the 30 kW and 60 kW generatorsturned on. Here, the timer is initialized to zero and is restarted.Then, after a suitable predetermined time, t_trans (e.g., withoutlimitation, 3 seconds; any suitable time), a “light” load state isentered at 70 with only the 30 kW generator turned on and the 60 kWgenerator turned off.

At 70, the timer is initialized to zero and is restarted. Then, after asuitable predetermined time, t_dwell (e.g., without limitation, 3seconds; any suitable time), execution resumes at 72. At 72, if thefrequency (freq)<0.980, then a transition state or “light to normal”state is entered at 74 with both of the 30 kW and 60 kW generatorsturned on. Here, the timer is initialized to zero and is restarted.Then, after a suitable predetermined time, t_trans (e.g., withoutlimitation, 3 seconds; any suitable time), the “normal” load state isreentered at 58 with only the 60 kW generator turned on and the 30 kWgenerator turned off. Otherwise, the test at 72 is repeated.

Example 7

The disclosed control strategy can be extended to a system 80 with anenergy storage system 82 as shown in FIG. 10, in order to allow a wideroperating range on the frequency droop lines 84 as shown in FIG. 6. Thisshows the results of control logic 86 (FIG. 7) with multiple dispatchingpoints and with the energy storage system 82.

As shown in FIG. 6, an example four efficiency bands are employed: (1) a90%-100% high efficiency band 88; (2) a 120% extended band 90, whichincludes BIM; (3) an 80%-100% high efficiency band 92; and (4) a 110%extended band 94, which includes BIM. At point 96, for example, at about36 kW load or 120% (36 kW) of the load, with 100% for the 30 kWgenerator plus 20% for BIM, the frequency is starting to be less thanthe specified frequency with BIM (e.g., f, pu, of 0.976). Here, the 60kW generator is turned on at 98. After a predefined time (t_trans), the30 kW generator is turned off at 99 to allow more load power tolerance.

Without BIM, the 30 kW generator is turned on at 100 or 100% (60 kW) ofthe load for the 60 kW generator. However, with BIM, the 30 kW generatoris turned on at 102 or 110% (66 kW) of the load, with 100% for the 60 kWgenerator plus 10% for BIM. The 30 kW generator is turned off at 104,which is 80% (48 kW) of the load for the 60 kW generator, or 53.33% (48kW) of the load for both of the 30 kW and 60 kW generators.

Example 8

FIG. 7 shows a flow chart of the time-based control logic 86corresponding to the results of FIG. 6. The logic 86 starts at 110.During initialization, at 112, the 60 kW generator is turned on, and the30 kW generator is turned off. A timer is initialized to zero and isstarted. Then, after a suitable predetermined time, t_init (e.g.,without limitation, 5 seconds; any suitable time), execution resumes at114, which is the “normal” load state (e.g., 30 kW to 60 kW in thisexample) with only the 60 kW generator turned on. Here, the timer isinitialized to zero and is restarted. Then, after a suitablepredetermined time, t_dwell (e.g., without limitation, 3 seconds; anysuitable time), execution resumes at 116.

At 116, if the frequency (freq)<0.980, and if at 118 extended BIM is notbeing used, then a “high” load state (e.g., 60 kW to 90 kW) is enteredwith both 30 kW and 60 kW generators turned on at 120. Here, the timeris initialized to zero and is restarted. Then, after a suitablepredetermined time, t_dwell (e.g., without limitation, 3 seconds; anysuitable time), execution resumes at 122. At 122, if the frequency(freq)>0.989, then the “normal” load state is reentered at 114 with onlythe 60 kW generator turned on and the 30 kW generator turned off.Otherwise, the test at 122 is repeated. On the other hand, if extendedBIM is being used at 118, then at 124, if the frequency (freq)<0.978,then the “high” load state is entered at 120, and, otherwise, the testat 116 is repeated.

On the other hand, if the test at 116 is false, then at 126, if thefrequency (freq)>0.991, then a transition state or “normal to light”state is entered at 128 with both of the 30 kW and 60 kW generatorsturned on. Here, the timer is initialized to zero and is restarted.Then, after a suitable predetermined time, t_trans (e.g., withoutlimitation, 3 seconds; any suitable time), a “light” load state isentered at 130 with only the 30 kW generator turned on and the 60 kWgenerator turned off Otherwise, if the test failed at 126, then the“normal” load state is reentered at 114.

At 130, the timer is initialized to zero and is restarted. Then, after asuitable predetermined time, t_dwell (e.g., without limitation, 3seconds; any suitable time), execution resumes at 132. At 132, if thefrequency (freq)<0.980, and if extended BIM is being used at 134, and ifthe frequency (freq)<0.976 at 136, then a transition state or “light tonormal” state is entered at 138 with both of the 30 kW and 60 kWgenerators turned on. Here, the timer is initialized to zero and isrestarted. Then, after a suitable predetermined time, t_trans (e.g.,without limitation, 3 seconds; any suitable time), the “normal” loadstate is reentered at 114 with only the 60 kW generator turned on andthe 30 kW generator turned off. On the other hand, if extended BIM isnot being used at 134, then the transition state or “light to normal”state is entered at 138. Otherwise, if the test failed at 136, then step132 is repeated.

On the other hand, if the frequency (freq) is not less than 0.980 at132, then at 140 it is determined if the frequency (freq)>0.996. If not,then the test at 132 is executed again. Otherwise, if the frequency(freq) is greater than 0.996, then an “inverter” state is entered at 142with both of the 30 kW and 60 kW generators turned off and power beingsupplied by BIM. Here, the timer is initialized to zero and isrestarted. Then, after a suitable predetermined time, t_inv (e.g.,without limitation, 3 seconds; any suitable time), it is determined at144 if the frequency (freq)<0.996. If so, then the “light” load state isreentered at 130. Otherwise, the “inverter” state is reentered at 142.

Steps 118,124,134,136 correspond to the use of battery management (withBIM) with the extended bands 90,94 of FIG. 6, which include BIM.

Example 9

FIG. 8 shows the results of control logic including a 90% efficiencyband 150 with multiple dispatching points 152,154,156,158,160, butwithout an energy storage system, in which combinations are shown for:(1) dispatching a 30 kW generator; (2) dispatching a 60 kW generator;(3) dispatching both 30 kW and 60 kW generators; (4) dispatching two 30kW generators and one 60 kW generator; and (5) dispatching three 30 kWgenerators and one 60 kW generator. For example, at 162, the 30 kWgenerator is turned on at 60 kW (100%) of the 60 kW generator. Also, at164, the 30 kW generator is turned off at 54 kW (60%) of the 30 kW and60 kW generators.

Example 10

FIG. 9 shows system efficiency versus load with: (1) a 30 kW generator170; (2) a 60 kW generator 172; (3) both 30 kW and 60 kW generators 174;(4) two 30 kW generators and one 60 kW generator 176; (5) three 30 kWgenerators and one 60 kW generator 178; and (6) four 30 kW generatorsand one 60 kW generator 180.

Example 11

FIG. 10 is a simplified block diagram of the system 80 including twoexample generators 200,202, the energy storage system 82 including aDC/AC inverter or BIM 204 and a suitable energy store, such as a numberof batteries 206. A grid 208 is formed by three example distributionnodes 210,212,214, and powers three example loads 216,218,220. In thisexample, the generators 200,202, the DC/AC inverter or BIM 204, the grid208, the distribution nodes 210,212,214, and the loads 216,218,220 arethree-phase devices, although the disclosed concept is applicable todevices which have any suitable number of phases. The distribution nodes210, 212 and 214 correspond to the respective generators 200, 202 andthe DC/AC inverter or BIM 204. Although, one load 216, 218 or 220 isoperatively associated with each of the distribution nodes 210,212,214,any suitable number of loads can be employed. A suitable communicationchannel 221 is provided for exchanging information between thegenerators 200,202, the DC/AC inverter or BIM 204, and the distributionnodes 210,212,214.

Example 12

FIG. 11 shows a control view for part of the system 80 of FIG. 10 inwhich a distribution manager (DM) 222,224 is provided for the respectivedistribution nodes 210,212. For convenience of illustration, the DC/ACinverter or BIM 204 and the corresponding distribution node 214 of FIG.10 are not shown. It is to be understood, however, that a correspondingDM (not shown) is provided for the distribution node 214. Also, the sameor different loads 216,218 or any suitable load can be employed in FIGS.10 and 11.

The DMs 222,224 and generators 200,202 cooperatively function toprovide: (1) adjustable load sharing by remote communications employinga communication module (COMM) 226 in each of the DMs 222,224 andgenerators 200,202; (2) automatic synchronization (AUTO SYNCH) 228 forhot plug-and-play of the corresponding generator 200 or 202; (3) remotestart/stop control 230 of the corresponding generator 200 or 202; and(4) load management control 232. Additional control is added to theconvention generator control 234 to provide generator functions for thedroop controller 52.

Moreover, the efficiency of the overall system 80 can be maximized. Thegenerators 200,202 are run at the highest efficiency most of the timefor both the loads 216,218,220 (FIGS. 10 and 11) and energy storagecharging through the energy storage system 82 (FIG. 10). The thresholdsof the extended band can also be adjusted to automatically depend on thestate of charge of the energy storage system 82. The controlarchitecture can easily be expanded to multiple distributed generatorsystems as were discussed, above, in connection with FIGS. 8 and 9.

The control strategy for load management is accomplished throughobservation of the frequency of the grid 208 (FIG. 10). Variations inload demand create measurable fluctuations in frequency which, in thedisclosed concept, are then associated with shedding of a number ofloads or prohibiting additional load, in order to prevent network powerquality issues. For example, based on default or configured loadpriorities, the control system sequentially dispatches loads based on apredetermined process accounting for network stability. The disclosedconcept preferably focuses on operation in an islanded mode, namely,with no utility grid or utility connection.

Example 13

Sources in a microgrid are operated with an overall power-frequencydroop 250 as shown in FIG. 12, which facilitates real power sharingbetween the various sources, such as the example generators 200,202, andthe DC/AC inverter or BIM 204 of FIG. 10. The various sources can havethe same droop parameters, such that each generator 200,202 shares poweraccording to its rating, for minimal fuel consumption, or may not havethe same droop parameters. As the load increases, the frequency drops.Therefore, frequency can be used as an indication of the system load.Overloads can be detected using frequency, as shown in the example inFIG. 12, where a measured frequency below 0.98 pu indicates that thesources are outputting more than 1 pu of rated power. This frequencybased load shedding preferably takes into account the frequency droop ofthe various sources during normal operation.

Example 14

In a microgrid with synchronous generators, such as 200 or 202 of FIG.10, driven by internal combustion engines (ICEs) (not shown),application or rejection of a load results in a transient frequencydrop. When a load is applied, more power is transferred from the rotor(not shown) than is input, and the rotor begins to slow down. Thegovernor (not shown) detects the change in speed and increases the powerto the rotor. Because of the relatively small inertia in the microgridconsidered, this transient frequency drop may be larger than the normaloperating frequency droop.

A load shedding algorithm 300 as shown in FIG. 13 preferably does notshed loads for transient frequency dips from which the system willrecover. In the event of a severe overload, the corresponding ICE (notshown) may stall. In order to deal with this potential problem,frequency based load shedding preferably acts relatively quickly in theevent of a severe overload, in order to prevent the stalling ofgenerators and the resulting system collapse. In the disclosedmicrogrid, the generators preferably employ, for example and withoutlimitation, diesel ICEs (not shown) which are over-rated by 50% to 100%in order to improve transient performance. In the case of an overloadwhich may exceed the rating of the electrical machine, but not cause thegenerator to stall, load shedding need not happen immediately. In thecase where demand based dispatching of generators is employed, the loadshedding algorithm 300 is preferably sufficiently slow in order thatanother source may be brought online before any loads are shed. However,in the case of an emergency (e.g., a severe overload), loads are shedimmediately in order to prevent the generators from stalling.

In accordance with the disclosed concept, an under-frequency loadshedding relay, or “F²t” relay measures the system frequency(SystemFreq) (F) 302 and compares it to a stress threshold(StressThresholdFreq) (F_(THRESH)) 304. The stress threshold 304 may bechosen, for example, to be the rated droop frequency (e.g., 0.98 pu inFIG. 12), or some other suitable value. The input value (i.e.,input=system frequency−stress threshold frequency) 306 is fed into asaturation block 308 having an output 310 such that:

-   -   output=0 if (system frequency−stress threshold frequency)>=0,        otherwise output=(system frequency−stress threshold frequency).        This output value is then fed through a function, such as a        squaring function 312. A leakage term 314 is subtracted by        subtraction function 316, and the result 318 is added to an        accumulator, or a discrete-time integration function, 320. The        accumulator 320 is normalized to a maximum value (not shown),        and different loads (e.g., 216,218,220 of FIG. 10, or 216,218 of        FIG. 11) can be assigned shedding thresholds by a priority load        shedding table 322 based upon the accumulator value 324, and the        loads can be shed by shedding commands 326 in the order of        priority.

The load shedding algorithm 300 can be executed, for example and withoutlimitation, by any suitable processor, such as the DG control 230 (FIG.11) for the intelligent distribution managers 222,224 (FIG. 11) or forthe distribution nodes 210,212,214 (FIG. 10), or by processor-basedloads, such as 216,218,220 of FIG. 10 or 216,218 of FIG. 11, which canindependently shed corresponding loads.

Example 15

For example, Table 1 shows high (H) and low (L) thresholds for sixexample normal loads (LP) and two example environmental control units(ECUs) (e.g., without limitation, air conditioning systems; HVAC;three-phase loads; loads which draw more current than normal loads).

TABLE 1 Load or Unit High or Low Threshold Threshold Value ECU1_H 0.25ECU1_L 0.10 LP1_H 0.35 LP1_L 0.25 LP2_H 0.40 LP2_L 0.30 LP3_H 0.45 LP3_L0.35 LP4_H 0.50 LP4_L 0.40 LP5_H 0.55 LP5_L 0.45 LP6_H 0.60 LP6_L 0.50ECU2_H 0.75 ECU2_L 0.60

In this example, the ECU1 will be disconnected first when theaccumulator value is larger than 0.25 and the ECU1 will be reapplied ifthe accumulator value 324 is less than 0.10. If the accumulator value324 is relatively very large (e.g., without limitation, 0.57), then theECU1, LP1, LP2, LP3, LP4 and LP5 will all be disconnected. Then, whenthe accumulator value 324 drops to, for example, 0.44, then LP5 will bereapplied.

Example 16

Many legacy generators are manually controlled and, as such, do not havethe ability to interface with an autonomous operating microgrid as isdisclosed herein. Hence, the interface of an intelligent distributionmanager (e.g., 222, 224 of FIG. 11) with integrated control is employed,which can easily be retrofit into legacy generating systems and linkwith a microgrid communication bus (e.g., 221 of FIG. 10). Such acontrol device needs to be able to interface to legacy DG, such as thegenerators 200,202 of FIG. 10, and the DC/AC inverter 204 in a simpleand direct manner. Existing generators have diverse control inputs andstatus outputs which are accessed by a microgrid grid controller. Thecontrol of a suitable intelligent distribution manager preferablyinterfaces to all of these DG and DER (Distributed Energy Resource)types.

The DG control 230 (FIG. 11) for the intelligent distribution manager222,224 (FIG. 10) employs voltage and frequency droop control, whichassures that the real and reactive power are shared equally among allthe generators 200,202, based solely on local measurements. By applyingdroop slopes for control, the power flow of the DG is determined by thepower demand of the loads 216,218,220. This ensures that the DG operatesat the lowest capacity necessary. Remote start/stop capability 230allows the microgrid intelligent distribution manager 222,224 toremotely turn on or turn off the corresponding generator 200,202. Thisfunctionality allows the DG to operate in a peak shaving mode when anenergy storage system, such as 82 of FIG. 10, is employed. Essentially,the DG is operated at full load to charge the energy storage system 82and supply the loads 216,218,220. When the energy storage system 82 isfully charged, the DG is turned off (e.g., the “inverter” state 142 ofFIG. 7), and the loads 216,218,220 are supplied solely or partially fromthe energy storage system 82. When the energy storage system 82 is belowa certain state-of-charge (SOC), the DG is turned on again, and theenergy storage system 82 is charged while simultaneously supplying theloads. This enables the DG to operate with maximum possible fuelefficiency within the certainty of the disclosed efficiency bands.

Example 17

The leakage term 314 of FIG. 13 acts to deplete the accumulator 320 oncethe load has decreased and the frequency is restored above the stressthreshold. Hysteresis, as shown above in Table 1, for example, is usedfor the reapplication of loads, based on the accumulator output 324.

Alternatively, frequency based load shedding can employ a suitably lowfrequency threshold, chosen to be lower than any expected transientfrequency dip for emergency load shedding (e.g., with no time delay),and a stress threshold with a time delay for non-emergency loadshedding. When the low frequency is observed, a number of the loads216,218,220 (FIG. 10) are shed because the generators 200,202 are in theprocess of stalling. For non-emergency load shedding, a delay can beused, in order that the system frequency remains below the stressthreshold for a predetermined period of time before loads are shed.Then, the loads can be shed sequentially until the frequency is restoredabove the stress threshold. The delay preferably accounts for the timeneeded for extra generation to be dispatched. In the case where extrageneration cannot be dispatched for whatever reason, load sheddingensures that the remaining loads may still be served, instead ofallowing the generators 200,202 to trip due to thermal overload.

Example 18

An example of load shedding is discussed in connection with FIG. 14.This shows a plot 350 of an example simulation of the disclosed “F²t”relay of the load shedding algorithm 300 of FIG. 13. In this simulation,one 30 kW diesel generator (not shown) and one 60 kW diesel generator(not shown) supply 70 kW 352 of load. At 5 seconds at 354, the 60 kWgenerator trips offline. This causes the remaining 30 kW generator,which has an over-rated diesel engine, to stall.

Example 19

FIGS. 15A-15C show respective plots 400,402,404 of a simulation of 30 kWand 60 kW diesel generators (not shown) operating, with the 60 kWgenerator tripping offline, thereby causing the 30 kW generator to beginto stall, and with low priority (LP) and HVAC loads being shed. In theplot 400 of FIG. 15A there is a plot 406 of generator frequency, in perunit, as well as a plot 408 of a filtered and sampled version of thefrequency used by the load shedding algorithm 300 of FIG. 13.

The plot 402 of FIG. 15B shows the output 324 of the normalizedaccumulator 320 of FIG. 13, and the output 313 of the (F−F_(THRESH))²function 312.

The plot 404 of FIG. 15C shows various loads being shed (HVAC 410, LP412), and after a few seconds, some loads (e.g., LP6 414) are reappliedafter the system recovers. The vertical lines represent the timing ofdigital signals (shedding commands 326 of FIG. 13) used to shed andrestore loads.

While specific embodiments of the disclosed concept have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the disclosedconcept which is to be given the full breadth of the claims appended andany and all equivalents thereof.

What is claimed is:
 1. A generator dispatching control method for amicrogrid, said method comprising: employing a plurality of generatorshaving a plurality of different rated capacities; employing a pluralityof distribution nodes; powering at least some of said distribution nodesby said generators; forming a grid by said distribution nodes, said gridincluding a system frequency; powering a plurality of loads by said gridthrough said distribution nodes, said loads having a power demand;operating by a processor said generators and said loads under transientconditions based upon a plurality of efficiency bands; and employingeach of said efficiency bands for a corresponding one of saidgenerators, each of said efficiency bands including a plurality ofgenerator switching points based upon droop of said system frequency andthe power demand of said loads.
 2. The generator dispatching controlmethod of claim 1 further comprising: powering at least some of saiddistribution nodes by an energy storage system.
 3. The generatordispatching control method of claim 2 further comprising: dynamicallymodifying said efficiency bands based on availability of said energystorage system.
 4. The generator dispatching control method of claim 2further comprising: supporting by said energy storage system, based uponsaid efficiency bands, at least one of the different rated capacities ofat least one of said generators; and delaying starting of another one ofsaid generators, when said loads exceed the at least one of thedifferent rated capacities of said at least one of said generators. 5.The generator dispatching control method of claim 2 further comprising:employing as said energy storage system an energy storage and a DC/ACinverter or battery interface module.
 6. The generator dispatchingcontrol method of claim 2 further comprising: employing an extendedefficiency band operatively associated with said energy storage system;and automatically adjusting thresholds of said extended efficiency bandto depend on a state of charge of said energy storage system.
 7. Amicrogrid power generation system comprising: a plurality of generatorshaving a plurality of different rated capacities; a plurality ofdistribution nodes, at least some of said distribution nodes beingpowered by said generators; a grid formed by said distribution nodes,said grid including a system frequency; a plurality of loads powered bysaid grid through said distribution nodes, said loads having a powerdemand; and a processor including a plurality of efficiency bands, eachof said efficiency bands being for a corresponding one of saidgenerators and including a plurality of generator switching points basedupon droop of said system frequency and the power demand of said loads,said processor being structured to operate said generators and saidloads under transient conditions based upon said efficiency bands. 8.The microgrid power generation system of claim 7 wherein at least one ofsaid distribution nodes is powered by an energy storage system.
 9. Themicrogrid power generation system of claim 8 wherein said processor isfurther structured to cause at least one of said generators to operateat or close to a corresponding at least one of the different ratedcapacities, thereby simultaneously supplying said loads and chargingsaid energy storage system, followed by turning off said generators andsupplying said loads from said energy storage system.
 10. The microgridpower generation system of claim 7 wherein said processor is furtherstructured to provide a predetermined response delay for said systemfrequency to settle at a corresponding operating point before turning onone of said generators.
 11. The microgrid power generation system ofclaim 7 wherein said processor is further structured to shed a number ofsaid loads if the system frequency droops at a rate exceeding apredetermined value, and to reconnect a number of said number of loadsfollowing recovery of said system frequency.
 12. The microgrid powergeneration system of claim 7 wherein said processor is furtherstructured to adjust said efficiency bands, a number of thresholds, anumber of frequency droop slopes and a number of delays.
 13. Themicrogrid power generation system of claim 8 wherein said processor isfurther structured to provide a first normal state for said loads and asecond state for said loads, said second state requiring less power thansaid first normal state, a first transition from said first normal stateto said second state, and a second transition from said second state tosaid first normal state.
 14. The microgrid power generation system ofclaim 13 wherein said processor is further structured to provide a thirdstate for said loads, said third state requiring greater power than saidfirst normal state, and a fourth inverter state; and wherein when saidprocessor turns off said generators, said energy storage system suppliespower to said loads.
 15. The microgrid power generation system of claim7 wherein said generators include a first generator having a first ratedcapacity of said different rated capacities and a second generatorhaving a second rated capacity of said different rated capacities, saidsecond rated capacity being two times greater than said first ratedcapacity; wherein said processor is further structured, when said secondgenerator is turned on, to turn on the first generator when the powerdemand of said loads is 100% of the second rated capacity of said secondgenerator, to turn off the first generator when the power demand of saidloads is less than 100% of the second rated capacity of said secondgenerator; wherein said processor is further structured, when saidsecond generator is turned on, to turn on the first generator when thepower demand of said loads is less than the first rated capacity of saidfirst generator and, after a predetermined time, to turn off the secondgenerator; and wherein said processor is further structured, when saidsecond generator is turned off, to turn on the second generator when thepower demand of said loads increases to the first rated capacity of saidfirst generator.
 16. A load shedding control method for a microgrid,said method comprising: employing a plurality of generators; employing aplurality of distribution nodes; powering at least some of saiddistribution nodes by said generators; forming a grid by saiddistribution nodes, said grid including a system frequency; powering aplurality of loads by said grid through said distribution nodes; andmeasuring the system frequency of said grid, comparing the measuredsystem frequency to a frequency threshold, and responsively shedding bya processor a number of said loads from said grid or prohibitingadditional load to said grid.
 17. The load shedding control method ofclaim 16 further comprising: providing at least one of: not sheddingsaid loads for transient frequency dips of the system frequency fromwhich the microgrid will recover, immediately shedding a number of saidloads in order to prevent stalling of a number of said generators, andpermitting the measured system frequency to be less than the frequencythreshold for a predetermined period of time before a number of saidloads are shed.
 18. The load shedding control method of claim 16 furthercomprising: defining an input value equal to the system frequency lessthe frequency threshold; providing an output having a value of zero ifthe input value is greater than or equal to zero, or otherwise saidinput value; feeding the value of said output through a function havinga value; subtracting a value from the value of said function to providea result; adding the result to an accumulator or a discrete-timeintegration function having a value; normalizing the value of theaccumulator to a maximum value; assigning said loads correspondingshedding thresholds; and shedding a number of said loads based upon thevalue of said accumulator and the corresponding shedding thresholds. 19.The load shedding control method of claim 16 further comprising:employing as said frequency threshold a rated droop frequency of one ofsaid generators.
 20. The load shedding control method of claim 18further comprising: employing as said function having the value asquaring function.
 21. A load shedding system comprising: a plurality ofgenerators; a plurality of distribution nodes, at least some of saiddistribution nodes being powered by said generators; a grid formed bysaid distribution nodes, said grid including a system frequency; aplurality of loads powered by said grid through said distribution nodes;and a processor including a frequency-based load shedding routinestructured to measure the system frequency of said grid, compare themeasured system frequency to a frequency threshold, and responsivelyshed a number of said loads from said grid or prohibit additional loadto said grid.
 22. The load shedding system of claim 21 wherein saidfrequency-based load shedding routine is further structured to define aninput value equal to the system frequency less the frequency threshold,provide an output having a value of zero if the input value is greaterthan or equal to zero, or otherwise said input value, feed the value ofsaid output through a function having a value, subtract a value from thevalue of said function to provide a result, add the result to anaccumulator or a discrete-time integration function having a value,normalize the value of the accumulator to a maximum value, assign saidloads corresponding shedding thresholds, and shed a number of said loadsbased upon the value of said accumulator and the corresponding sheddingthresholds.
 23. The load shedding system of claim 21 wherein said gridis a microgrid operating in an islanded mode.
 24. The load sheddingsystem of claim 21 wherein the frequency-based load shedding routine isfurther structured to provide emergency load shedding to immediatelyshed a number of said loads in order to prevent a number of saidgenerators from stalling, and non-emergency load shedding.
 25. The loadshedding system of claim 21 wherein the frequency-based load sheddingroutine is further structured to not shed said loads for transientfrequency dips of said system frequency from which the load sheddingsystem will recover.
 26. The load shedding system of claim 21 whereinthe frequency-based load shedding routine is further structured toimmediately shed a number of said loads in order to prevent stalling ofa number of said generators.
 27. The load shedding system of claim 21wherein the frequency-based load shedding routine is further structuredto permit the measured system frequency to be less than the frequencythreshold for a predetermined period of time before a number of saidloads are shed.
 28. The load shedding system of claim 27 wherein aftersaid predetermined period of time, said number of said loads are sheduntil said system frequency is greater than the frequency threshold. 29.The load shedding system of claim 27 wherein said predetermined periodof time permits dispatching of extra generation.
 30. The load sheddingsystem of claim 22 wherein the frequency-based load shedding routineincludes a table having a high threshold value and a low threshold valuefor each of said loads; wherein a corresponding one of said loads isshed when the value of said accumulator is greater than the highthreshold value of the corresponding one of said loads; and wherein thecorresponding one of said loads is reapplied to said grid when the valueof said accumulator is less than the low threshold value of thecorresponding one of said loads.
 31. The load shedding system of claim21 wherein one of said distribution nodes is powered by an energy storeand a DC/AC inverter or battery interface module.